Loans and Leases (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 6,220,000		$ 6,220,000		$ 6,080,000
Advances from Federal Home Loan Banks	128,700		128,700		835,200
Federal Home Loan Bank Advances, Current Borrowing Capacity	6,090,000		6,090,000		5,240,000
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	1,200		1,200		1,700
Loans and Leases Receivable, Gross, Carrying Amount, Covered	62,100		62,100
Federal Reserve Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	2,170,000		2,170,000		2,080,000
Loans held for sale	58,961		58,961		51,179
Transfer of Portfolio Loans and Leases to Held-for-sale			0	$ 84,509
Proceeds From Sales of Loans	164,228	$ 146,954	302,766	342,085
Proceeds from sales of portfolio loans			0	32,294
Gain (Loss) on sale of loans and leases			0	164
Non-PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccruing	85,055		85,055		92,534
Accretion Income	4,100	3,000	7,000	$ 6,000
PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccruing	1,600		1,600		600
Accretion Income	14,200	$ 20,400
HomeBancorp Inc.
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	7,000		7,000
Guaranty Bank
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	12,300		12,300		14,200
Cordia Bancorp Inc.
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	1,900		1,900		2,700
Bancorporation
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	14,700		14,700		18,100
Federal Home Loan Bank of Atlanta
Financing Receivable, Recorded Investment [Line Items]
Loans Pledged as Collateral	9,080,000		9,080,000		8,750,000
Federal Reserve Bank
Financing Receivable, Recorded Investment [Line Items]
Loans Pledged as Collateral	$ 2,880,000		$ 2,880,000		$ 2,770,000